[Hexagon] Remove USR_OVF from CtrRegs register class

USR_OVF is a subregister of USR, which is a member of CtrRegs. Having both
a register and its proper subregister in the same register class has bad
consequences for lane mask calculation: based solely on the lane mask info,
USR_OVF would not appear to be a subregister of USR.

llvm-svn: 282192